For most electronic devices, reliable and stable power is necessary. For maintaining desirable performance of the power system, a current sensor is widely used to sense the current status of the power system. Moreover, the current sensor may be applied to other electronic circuits in order to achieve desirable performance of the electronic circuits.
Generally, the current sensors are classified into two types, i.e. a resistor-based current sensor and a current sensing transformer.
Referring to FIG. 1, a schematic circuit configuration of a resistor-based current sensor is illustrated. For measuring the current I passing through the test circuit, a test resistor R is connected to the test circuit in series and a potential difference V across the test resistor R is measured. According to Ohm's law, the current I passing through the test resistor R is directly proportional to the potential difference applied across the test resistor R (i.e. I=V/R). The resistor-based current sensor is simple for current measurement. When current flows through the test resistor R, the electrical energy will be converted to heat and thus the temperature of the test resistor R is increased. The increased temperature of the test resistor R may result in a substantial error of the measured current and thus a high power loss.
Referring to FIG. 2, a schematic circuit configuration of a current sensing transformer is illustrated. For measuring the current I passing through the test circuit, the test circuit is connected to a primary winding coil of the current sensing transformer in series. As known, the ratio of the primary amperage (I) to the secondary amperage (Iout) is in an inverse proportion to the ratio of the primary turn (Np) to the secondary turn (Ns). That is, the current I passing through the test circuit is deduced from an equation: i.e. I=Iout×(Ns/Np).
The current sensing transformer, however, is applicable to a single loop circuit. For measuring the currents passing through two loop circuits, two current sensing transformers are required.
Please refer to FIG. 3(a), which is a schematic circuit configuration illustrating two current sensing transformers used for measuring the currents of two loop circuits. A first loop circuit Lp1 is connected to a primary winding coil of the first current sensing transformer CT1 in series. A second loop circuit Lp2 is connected to a primary winding coil of the second current sensing transformer CT2 in series. When a first test current Ip1 passes through the first loop circuit Lp1, a first output current Is1 passing through the secondary winding coil of the first current sensing transformer CT1 is measured. Likewise, when a second test current Ip2 passes through the second loop circuit Lp2, a second output current Is2 passing through the secondary winding coil of the second current sensing transformer CT2 is measured. The first test current Ip1 and the second test current Ip2 may be deduced from the equations: Ip1=Is1×(Ns1/Np1) and Ip2=Is2×(Ns2/Np2), respectively. In these equations, Np1 is the turn number of the primary winding coil of the first current sensing transformer CT1, Ns1 is the turn number of the secondary winding coil of the first current sensing transformer CT1, Np2 is the turn number of the primary winding coil of the second current sensing transformer CT2, and Ns2 is the turn number of the secondary winding coil of the second current sensing transformer CT2. The currents Ip1, Is1, Ip2 and Is2 are plotted in FIG. 3(b). As known, using two current sensing transformers for measuring the currents of two loop circuits is not cost-effective. In addition, the overall volume and the overall weight of the electronic device are increased.
In views of the above-described disadvantages resulted from the prior art, the applicant keeps on carving unflaggingly to develop a current sensing transformer according to the present invention through wholehearted experience and research.